Excitonic Complexes in Wide-Gap II-VI Semiconductors

Abstract

During the last years it became evident that excitonic complexes play a very important role concerning the nonlinear optical properties of wide-gap II–VI semiconductors. Thus, tremendous efforts have been made to find out and describe excited states of free and, particularly, bound-exciton complexes, their excitation and decay mechanisms, and related scattering phenomena. This paper surveys recent results regarding bound-exciton complexes and their interaction with free-exciton systems (biexcitons) in wide-gap II–VI materials at moderate to high excitation densities. Through means of resonant excitation spectroscopy it is shown that bound-exciton systems in II–VI’s always possess a manifold of excited electronic states in which optical transitions often only become allowed under intense light irradiation. A comparison of spectroscopic investigations at CdS, ZnO, and ZnS is presented which allows for the development of comprehensive term schemes for neutral-impurity-exciton complexes in II–VI’s. Mainly three types of excited states exist for bound-exciton complexes: (i) states which are described through electronic excitation of one electron or hole, while the other particles involved remain in their single-particle ground states, (ii) states which result from the participation of holes from lowerlying valence bands, and (iii) vibronic or rotational excited states. Measurements in magnetic fields up to 15 T allow for a well-founded assignment of optical transitions to these states. Additionally, exciton complexes give rise to resonant phononic or electronic Raman scattering processes which often break symmetry selection rules and provide further information on the levels involved. The energies and Stokes shifts of the scattered lines sensitively depend on quantities as complexes extension and binding energy.